An air cleaner unit for an internal combustion engine is, for example, described in Japanese Unexamined Patent Application Publication No. 7-247923 (U.S. Pat. No. 5,569,311). The described air cleaner unit includes a casing and a filter element housed in the casing. The casing is constructed of a case body and a cap.
The case body has an opening and an element housing portion at the opening. The case body is disposed such that the opening is open in an upward direction of a vehicle. The cap is coupled to cover the opening of the case body with an opening thereof. The filter element is housed in the casing such that a peripheral portion of the filter element is interposed between a sealing surface of the case body and a sealing surface of the cap. The sealing surfaces of the case body and the cap are formed on peripheries of the openings thereof. Further, the peripheral portion of the filter element is compressed and deformed between the sealing surfaces of the case body and the cap for sealing the case body and the cap. As such, the filter element is held in the casing in a horizontal position that is perpendicular to an up and down direction of the vehicle.
Also, it is proposed to arrange an air cleaner unit such that a filter element is held in a vertical position for a reason of mounting in a vehicle. For example, due to the reason of mounting in a vehicle, a case body is disposed at a front end portion of the vehicle such that its opening opens in a vehicle rearward direction. In this case, the case body and the cap are coupled through a hinge part disposed at bottom portions of the case body and the cap for easing coupling work. However, because the hinge part is likely to be hidden or almost hidden by the case body and peripheral engine functional parts, it will be difficult to properly engage the hinge part. Also, if the engagement of the hinge part is insufficient, it is difficult to properly couple the cap to the case body such that a peripheral portion of a filter element is appropriately interposed between the sealing surfaces of the case body and the cap.